The present invention relates to a method and system for operating an internal combustion engine at maximum output torque under varying operating conditions by oscillating the setting of ignition timing on each side of a variable reference setting.
Spark ignition timing is usually controlled in response to engine variables such as crankshaft speed and intake vacuum so that the engine output torque is at maximum while fuel consumption rate is a minimum in so far as it causes no problem with respect to knocking and exhaust gas emissision. However, due to difficulties inherent in calibrating engines to their particular operating characteristics during manufacture, ignition advance setting is not necessarily adjusted to the optimum value of each engine. Even if such calibration is achieved, the operating characteristics of the engine tend to vary as a function of environmental factors and aging, the particular ignition setting would cease to be the optium setting.
A spark ignition control system, shown and described in U.S. Pat. No. 3,142,967 granted to Paul H. Schweitzer, discloses means for dithering or oscillating the ignition setting on either side of a variable reference setting to detect the resultant variation in engine output variable such as engine speed at two consecutive points in time. The reference advance timing is corrected in one way or another in response to the result of comparison between the detected output engine variables. If the engine speed variation resulting from an ignition dithered in the advance direction is greater than that resulting from a subsequent ignition dithered in the retard direction, the reference setting is advanced by a predetermined amount, and if the situation is reversed the reference setting is retarded. The process is repeated so that ignition timing is maintained at an optimum position at which the engine delivers maximum output power. Since the corrective indications are derived from two successive engine output variables, the prior art system has an inherent difficulty in distinguishing between the variations caused by dither actions and those caused by manual acceleration or deceleration.
U.S. Pat. No. 4,379,333, assigned to the same assignee as the present invention, discloses an improved ignition control system in which the variation of engine output variable is detected at three successive points and compared one against each other to determine one of two specified conditions which occur exclusively when the ignition setting is dithered. Since the ignition setting is dithered on each side of a variable reference setting during each oscillation, there is occasioned a large amount of engine speed variation with a resultant poor drivability.